The present invention relates to an optical element, an optical system having such an optical element and an endoscope using such an optical system. More particularly, the present invention relates to an image formation optical system or a projection optical system having a function of forming an image around an axis of rotational symmetry as an annular image to an image pickup device.
Patent Document 1 listed below describes a known image pickup optical system having a refraction optical system, a reflection optical system and an image formation optical system arranged therein along with two optical paths so as to be capable of picking up a panoramic image and an axial image. Patent Document 2 describes a known endoscope similarly having two optical paths. Further, Patent Document 3 describes a known endoscope by means of which it is possible to omni-directionally observe the surroundings and Patent Document 4 describes a known capsule endoscope by means of which it is also possible to omni-directionally observe the surroundings. Furthermore, Patent Document 5 describes an image pickup device that can shoot omni-directionally the surroundings and also only forward at the same time. Finally, Patent Documents 6 through 13 describe an omni-directional optical system.    [Patent Document 1] Jpn. PCT National Publication No. 2003-042743    [Patent Document 2] U.S. Patent Application Publication No. 2004-0254424    [Patent Document 3] JP-A-60-42728    [Patent Document 4] JP-A-2001-174713    [Patent Document 5] JP-A-2002-341409    [Patent Document 6] U.S. Pat. No. 3,505,465    [Patent Document 7] U.S. Pat. No. 5,854,713    [Patent Document 8] U.S. Pat. No. 6,115,193    [Patent Document 9] U.S. Pat. No. 6,175,454    [Patent Document 10] U.S. Pat. No. 6,356,296    [Patent Document 11] U.S. Pat. No. 6,392,687    [Patent Document 12] U.S. Pat. No. 6,449,103    [Patent Document 13] U.S. Pat. No. 6,597,520